An image sensor generally includes an array of pixel cells, which include photo-conversion devices for converting light incident on the array into electrical signals, and peripheral circuitry, which includes circuitry for controlling devices of the array and circuitry for converting the electrical signals into a digital image.
In integrated circuit (IC) fabrication, it is often necessary to isolate semiconductor devices formed in the substrate. This is true for many types of ICs including, for example, DRAM, flash memory, SRAM, microprocessors, DSP and ASICs. The individual pixels of a Complementary Metal Oxide Semiconductor (CMOS) image sensor IC also need to be isolated from each other and from other devices.
A CMOS image sensor IC includes a focal plane array of pixel cells, each one of the cells including a photogate, photoconductor, or photodiode overlying a charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a floating diffusion node and a transistor, for resetting the diffusion node to a predetermined charge level prior to charge transfer. The pixel cell may also include a source follower transistor for receiving and amplifying a charge level from the diffusion node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the floating diffusion node. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS image sensor pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate. A photon impinging on a particular pixel of a photosensitive device may diffuse to an adjacent pixel, resulting in detection of the photon by the wrong pixel, i.e. cross-talk. Therefore, CMOS image sensor pixels must be isolated from one another to avoid pixel cross talk. In the case of CMOS image sensors, which are intentionally fabricated to be sensitive to light, it is advantageous to provide both electrical and optical isolation between pixels.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256.times.256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe operation of conventional CMOS image sensors, the contents of which are incorporated herein by reference.
FIG. 1 is a block diagram of a typical CMOS image sensor IC 100 with circuitry formed at a surface of a substrate 101. Image sensor 100 includes an array of pixel cells 111. The pixel cells (not shown) are arranged in columns and rows. Each pixel cell includes a photo-conversion device. As is known in the art, a pixel cell functions by receiving photons of light and converting those photons into charge carried by, for example, electrons. In order to produce an accurate and higher quality image, a photo-conversion device of a pixel cell should receive only photons from an imaged scene. Further, a pixel cell should not receive electrons that do not result from photoconversion.
After pixel cells of array 111 generate charge in response to incident light, electrical signals indicating charge levels are read out and processed by circuitry peripheral to array 111. Peripheral circuitry of image sensor 100 typically includes row select circuitry and column select circuitry (not shown) for activating particular rows and columns of array 111. Image sensor 100 also includes analog signal processing circuitry 112, analog-to-digital conversion circuitry 113, and digital logic circuitry 114. Circuitry 112, 113, and 114 can be on a same chip 101 as array 111, as shown in FIG. 1, or on a different chip. The analog signal processing circuitry 112 samples data from array 111 and analog-to-digital conversion circuitry 113 converts the analog signals sampled by circuitry 112 into digital signals. Digital logic circuitry 114 processes the digital signals to output a digital image representative of the light incident on array 111.
During operation, the peripheral circuitry, especially analog-to-digital conversion circuitry 113, generates photons and charge carriers, e.g. electrons. Most photons generated by peripheral circuitry are in the red to near infrared region. As these photons have a long wavelength, they are able to travel far in the substrate 101. If the peripheral circuitry is on the same chip 101 as array 111, photons and electrons generated by the peripheral circuitry can travel to and interfere with pixel cells of array 111, especially those pixel cells on the edges of array 111 adjacent to the peripheral circuitry.
FIG. 2 is an image 120 sensed under dark conditions by an IC similar to image sensor 100, discussed above in connection with FIG. 1. Typically, image sensor 100 includes a Bayer pattern color filter array such that image sensor 100 includes one subset of pixel cells for receiving blue light, one subset for receiving red light, and two subsets for receiving green light. Each quadrant 121, 122, 123, 124 of the image 120 shows light intensity sensed by a respective one of the four subsets of pixel cells. As shown in FIG. 2, the pixel cells near the edges of the array 111 will appear brighter than the pixel cells in the middle of array 111 because of interference by photons and electrons from the peripheral circuitry.
One method to reduce interference provides, around the array 111, a doped layer 115 (FIG. 1) in substrate 101, typically a highly doped p-type layer. Layer 115, however, still allows a majority of electrons to pass through. Additionally, at a typical width of 6 μm, few of the long wavelength (red and infrared) photons generated by the peripheral circuitry will be absorbed by layer 115.
Accordingly, it would be advantageous to have improved techniques for isolating an array of pixel cells from peripheral circuitry.